The magnetic properties of GdN thin films with bulk and 2.4% extended lattice parameters have been studied by x-band ferromagnetic resonance (FMR) spectroscopy. Simple, anisotropic, single line FMR spectra are observed in the 0 to 18 kOe field range. Both films show an in-plane easy axis of magnetization due to the predominance of the shape related demagnetization fields. The films deposited on (100) Si substrates are strained with uniaxial anisotropy constants up to K-u=+5x10(6) erg/cm(3) at T=4 K. The lattice extension reduces the magnetization and decreases the Curie temperature from 69 K for the bulk films to 20 K for the films with extended lattice constants. Highly increased FMR linewidths are observed for the lattice extended films indicating large, local variations of the demagnetization fields.